It's the iron lady of screen acting who provides virtually all the oomph in The Iron Lady.

That's because the subject of the film hardly comes into any kind of productive focus even as we marvel at the performer painting the portrait. This is a bad biography but a shimmering showcase.

Meryl Streep portrays former British prime minister Margaret Thatcher in the biographical drama about not only the only woman to hold the post, but the person who held it the longest.

Set in the present, from which staunchly conservative, fragile eightysomething Thatcher, suffering from Alzheimer's, looks back on her storied life, The Iron Lady uses flashbacks to depicts Thatcher's life from 1979 to 1990, and includes her battles with dementia and her continued occasional perception that she is still the prime minister.

Streep disappears into her role, as is her wont. Her "impersonation" is uncanny, from her look to her posture to her gestures to her gait to her distinct vocal delivery to the precision of her idiosyncratic line readings.

If Streep is to take home an Academy Award for her work here -- which, given her track record, the competition, and the current level of buzz, is certainly in the realm of possibility -- it will rival Al Pacino winning the Oscar for Best Actor for his performance in 1992's mediocre Scent of a Woman as a textbook example of a performance transcending the movie it emerges from.

Jim Broadbent, as the ghost of Thatcher's late husband, Denis, turns up at critical junctures to hear her out in her time-traveling confusion and participate in imagined conversations. This thrust of the film -- her touching hallucinatory sessions with Denis -- is when the film is at its best. Unlike Thatcher, who sees herself as preferring thoughts and ideas to feelings, the film itself seems to operate the opposite way.

Director Phylidda Lloyd, who also directed Streep in Mamma Mia!, gets a terrific performance from her lead -- so, at the very least, she deserves credit for putting Streep on the high wire and then staying out of her way. But the director fails to iron out the other script and production problems that keep us from feeling anything more than appreciation of Streep's thespian skills.

The screenplay by Abi Morgan, who also scripted the recent Shame, is based on a number of sources and includes material provided by a number of anonymous civil servants and politicians. Thatcher's career highlights (or lowlights, depending on your political vantage point) are touched on, but political neutrality seems to be the objective: Thatcher is neither lionized nor demonized politically.

But the film pays a price for spending all that time on Thatcher's dotage: we exit having learned very little about her vital political career.

Because of the fancifully impressionistic and episodic structure -- triggering its flashbacks with recent news events that initiate Thatcher's memories of vaguely connected parallel occurrences in the 1980s -- the film is certainly not trying to be a linear biopic, but neither does it substitute anything editorially interesting enough or artistically bold enough to fill in the blanks that we feel surrounded by.

The scattered film keeps us at far too much of a distance, lacking the kind of continuity and context that would allow us to feel illuminated. Instead, we find ourselves merely admiring a woman who succeeded substantially as a tough-minded politician in a man's world.

In a nutshell, Streep's bifurcated portrait -- abetted by persuasive makeup that puts the counterpart element in J. Edgar to shame -- looks, sounds, and seems fully realized, while the movie built around her sputters as it lurches from scene to scene.

The Iron Lady is both uneven and uninvolving. But for the brilliant central performance, it's a Streep uphill climb.